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Subject[PATCH 12/12] megaraid_sas: Convert SHT->queuecommand() to run host_lock-less
Date
From: Nicholas Bellinger <nab@linux-iscsi.org>

This patch converts megasas_queue_command_lck -> to megasas_queue_command
running in modern host_lock-less mode. This includes running using
struct megasas_instance->hba_lock to disable interrupts around existing
callers in megasas_queue_command(). In the existing code the
megasas_instance->hba_lock is taken to disable interrupts for

*) Setup of megasas_cmd for scsi_cmnd descriptor in megasas_get_cmd()

*) Issuing megasas_cmd to firmware via instance->instancet->fire_cmd()

Along with commit 1cb8d64a19c and 0fbb4eb3b to make the following
megasas_instance members use proper atomic_t values:

*) fw_issuepend_done: used to signal SCSI_MLQUEUE_HOST_BUSY during
->queuecommand(), and used during shutdown)

*) fw_outstanding: used to track number outstanding megasas_cmd
descriptors

The one part of the code not originaly held, but has now been converted
to hold instance->hba_lock w/ interrupts disabled megasas_build_ldio()
and megasas_build_dcdb(). This part of code was originally protected
by Scsi_Host->host_lock disabling interrupts.

This allows for megaraid_sas to run in host_lock less dispatch
w/ disabling interrupts around internal megasas_instance->hba_lock.
So far this is functioning with QEMU Megasas 8708EM2 HBA emulation
with SG_IO into KVM Guest from TCM_Loop backstores on a .37-rc3 KVM
x86_64 Host.

Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
---
drivers/scsi/megaraid/megaraid_sas.c | 68 ++++++++++++++++++++++-----------
1 files changed, 45 insertions(+), 23 deletions(-)
diff --git a/drivers/scsi/megaraid/megaraid_sas.c b/drivers/scsi/megaraid/megaraid_sas.c
index da0d3e3..db3075d 100644
--- a/drivers/scsi/megaraid/megaraid_sas.c
+++ b/drivers/scsi/megaraid/megaraid_sas.c
@@ -122,19 +122,17 @@ megasas_complete_cmd(struct megasas_instance *instance, struct megasas_cmd *cmd,
u8 alt_status);

/**
- * megasas_get_cmd - Get a command from the free pool
+ * __megasas_get_cmd - Get a command from the free pool
* @instance: Adapter soft state
*
* Returns a free command from the pool
+ * Locking: Called with instance->cmd_pool_lock w/ spin_lock_irqsave
*/
-static struct megasas_cmd *megasas_get_cmd(struct megasas_instance
- *instance)
+static struct megasas_cmd *
+__megasas_get_cmd(struct megasas_instance *instance)
{
- unsigned long flags;
struct megasas_cmd *cmd = NULL;

- spin_lock_irqsave(&instance->cmd_pool_lock, flags);
-
if (!list_empty(&instance->cmd_pool)) {
cmd = list_entry((&instance->cmd_pool)->next,
struct megasas_cmd, list);
@@ -143,7 +141,25 @@ static struct megasas_cmd *megasas_get_cmd(struct megasas_instance
printk(KERN_ERR "megasas: Command pool empty!\n");
}

+ return cmd;
+}
+
+/**
+ * megasas_get_cmd - Get a command from the free pool
+ * @instance: Adapter soft state
+ *
+ * Returns a free command from the pool
+ */
+static struct megasas_cmd *
+megasas_get_cmd(struct megasas_instance *instance)
+{
+ struct megasas_cmd *cmd;
+ unsigned long flags;
+
+ spin_lock_irqsave(&instance->cmd_pool_lock, flags);
+ cmd = __megasas_get_cmd(instance);
spin_unlock_irqrestore(&instance->cmd_pool_lock, flags);
+
return cmd;
}

@@ -1332,9 +1348,18 @@ megasas_dump_pending_frames(struct megasas_instance *instance)
* megasas_queue_command - Queue entry point
* @scmd: SCSI command to be queued
* @done: Callback entry point
+ *
+ * This is the main Linux/SCSI per I/O cmd dispatcher that is running
+ * in 'host_lock-less' mode w/o the legacy Scsi_Host->host_Lock held,
+ * and with IRQs enabled.
+ *
+ * This means that interaction with FW requires the instance->hba_lock
+ * be held with interrupts disabled, or that other megasas_instance
+ * reference must be done in an atomic fashion in modern host_lock-less
+ * mode.
*/
static int
-megasas_queue_command_lck(struct scsi_cmnd *scmd, void (*done) (struct scsi_cmnd *))
+megasas_queue_command(struct Scsi_Host *sh, struct scsi_cmnd *scmd)
{
u32 frame_count;
struct megasas_cmd *cmd;
@@ -1347,23 +1372,18 @@ megasas_queue_command_lck(struct scsi_cmnd *scmd, void (*done) (struct scsi_cmnd
if (atomic_read(&instance->fw_issuepend_done) == 0)
return SCSI_MLQUEUE_HOST_BUSY;

- spin_lock_irqsave(&instance->hba_lock, flags);
- if (instance->adprecovery != MEGASAS_HBA_OPERATIONAL) {
- spin_unlock_irqrestore(&instance->hba_lock, flags);
- return SCSI_MLQUEUE_HOST_BUSY;
- }
-
- spin_unlock_irqrestore(&instance->hba_lock, flags);
-
- scmd->scsi_done = done;
- scmd->result = 0;
-
if (MEGASAS_IS_LOGICAL(scmd) &&
(scmd->device->id >= MEGASAS_MAX_LD || scmd->device->lun)) {
scmd->result = DID_BAD_TARGET << 16;
goto out_done;
}

+ spin_lock_irqsave(&instance->hba_lock, flags);
+ if (instance->adprecovery != MEGASAS_HBA_OPERATIONAL) {
+ spin_unlock_irqrestore(&instance->hba_lock, flags);
+ return SCSI_MLQUEUE_HOST_BUSY;
+ }
+
switch (scmd->cmnd[0]) {
case SYNCHRONIZE_CACHE:
/*
@@ -1371,14 +1391,17 @@ megasas_queue_command_lck(struct scsi_cmnd *scmd, void (*done) (struct scsi_cmnd
* No need to send it down
*/
scmd->result = DID_OK << 16;
+ spin_unlock_irqrestore(&instance->hba_lock, flags);
goto out_done;
default:
break;
}

- cmd = megasas_get_cmd(instance);
- if (!cmd)
+ cmd = __megasas_get_cmd(instance);
+ if (!cmd) {
+ spin_unlock_irqrestore(&instance->hba_lock, flags);
return SCSI_MLQUEUE_HOST_BUSY;
+ }

/*
* Logical drive command
@@ -1387,6 +1410,7 @@ megasas_queue_command_lck(struct scsi_cmnd *scmd, void (*done) (struct scsi_cmnd
frame_count = megasas_build_ldio(instance, scmd, cmd);
else
frame_count = megasas_build_dcdb(instance, scmd, cmd);
+ spin_unlock_irqrestore(&instance->hba_lock, flags);

if (!frame_count)
goto out_return_cmd;
@@ -1414,12 +1438,10 @@ megasas_queue_command_lck(struct scsi_cmnd *scmd, void (*done) (struct scsi_cmnd
out_return_cmd:
megasas_return_cmd(instance, cmd);
out_done:
- done(scmd);
+ scmd->scsi_done(scmd);
return 0;
}

-static DEF_SCSI_QCMD(megasas_queue_command)
-
static struct megasas_instance *megasas_lookup_instance(u16 host_no)
{
int i;
--
1.7.3.4


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